This invention relates to a method and an apparatus for evaluating an epitaxial wafer for a light emitting device, a recording medium readable by a computer, and an epitaxial wafer for a light emitting device. More particularly, the invention relates to an epitaxial wafer useful for providing InGaAlP LEDs excellent in emission characteristics, and an examination method for examining emission characteristics of light emitting devices such as LED and laser diodes, and especially to an examination method capable of precisely evaluating crystallographic properties and emission efficiency of light emitting devices in form of wafers prior to making electrodes thereon.
Along with recent developments of the technology for fabricating films by MOCVD (metal organic chemical vapor deposition), it has become possible to prevent undesirable entry of impurities like oxygen and carbon and crystalline defects along the interface of epitaxial layers (epitaxially grown semiconductor layers), which adversely affect the emission characteristics of LED (light emitting diode), and to manufacture high-luminescence LED by mass-production.
However, it has been difficult heretofore to stably manufacture chips with a high emission efficiency for LED and other light emitting devices because dopants spread in the active layer invited a decrease in luminescence in initial stages of emission and deterioration by electrical conduction.
Regarding InGaAlP LED using Zn (zinc) as its p-type dopant, it has been known that carriers are trapped by a deep level made by Zn diffused into its active layer and large affect its emission characteristics. This phenomenon is explained in, for example, Jpn. J. Appl. Phys. Vol. 33 (1994) pp. L857xcx9cL859 xe2x80x9cEffect of Substrate Microrientation and Zn Doping Characteristics on the Performance of AlGaInP Visible Light-emitting Diodesxe2x80x9d, and Solid-State Electron. Vol. 38, No. 2, pp. 305xcx9c308. 1995 xe2x80x9cAlGaInP Orange Light-emitting-diodes Grown on Misoriented p-GaAs Substratesxe2x80x9d.
Although Zn has been known as adversely affecting the emission characteristics, relationship between quantities of Zn in the active layer and emission characteristics has not been clarified. In order to fabricate LED epitaxial wafer (wafer with a semiconductor layer epitaxially grown thereon) with good emission characteristics, the diffusion amount of Zn into the active layer should be controlled. However, it is still difficult to precisely control diffusion of Zn in the epitaxial process, and there is no easy measure for evaluating the diffusion amount of Zn. Therefore, it has been difficult to fabricate epitaxial wafers stable in emission characteristics.
That is, heretofore, it was difficult to control diffusion of Zn (zinc) as a dopant, and it was not clear which amount of diffusion of Zn into the active layer deteriorates the emission characteristics.
That is, because of insufficient researches on the relation of the diffusion amount of Zn into the active layer and the emission characteristics, no clear guidance has been given yet regarding the diffusion amount of Zn into the active layer.
To measure the amount of Zn diffused into an active layer, SIMS (secondary ion mass spectroscopy) is typically used. However, this method of evaluation requires a specialized technique. Moreover, it is essentially a destructive inspection, and cannot cope with individuals of wafers which are manufactured in a routine manner.
Conventional techniques involved another problem that emission efficiency could not be measured precisely and easily. That is, although emission efficiency is one of most important factors in optical characteristics of semiconductor light emitting devices, conventional methods were configured to measure the emission efficiency by cutting out a chip from an epitaxial wafer grown in liquid or vapor phase and having formed electrodes. Needless to say, this method requires the process of cutting out the chip. Therefore, it takes much time for cutting out a chip, and quick evaluation of emission efficiency is impossible. Moreover, since a wafer was cut out for evaluation, the production yield inevitably decreased.
As a method for quickly obtaining emission efficiency without touching the subject to be evaluated, there has been proposed an evaluating method using photoluminescence (PL). This is a method for measuring emission efficiency by measuring the lifetime of photoluminescence. For example, Japanese Patent Application No. hei 6-202296 discloses a method for LED having an AlGaAs active layer. Japanese Patent Application No. hei 3-279777 proposes an apparatus combining a probe and an XY stage to measurer such lifetime. Japanese Patent Application No. hei 5-196419 discloses a method for measuring such lifetime in InGaAlP LED and AlGaAs LED by laser excitation.
The reason why emission efficiency can be measured through measurement of lifetime can be explained as follows. Emission efficiency can be expressed by the product of internal quantum efficiency and external quantum efficiency (take-out efficiency). Since the external quantum efficiency may be considered constant among light emitting devices common in structure of the epitaxial layer, electrode pattern and package, emission efficiency depends on the internal quantum efficiency.
Lifetime xcfx84n can be expressed by the following equation from emitting lifetime xcfx84r and non-radiative lifetime.
1/xcfx84n=1/xcfx84r+1/xcfx84nrxe2x80x83xe2x80x83(1)
The internal quantum efficiency can be expressed by the following equation by using the lifetime.
xcex7i=Axc3x97xcfx84n/xcfx84r (A is a constant)xe2x80x83xe2x80x83(2)
It is noted from Equation (2) that emission efficiency is proportional to the lifetime xcfx84n. Therefore, emission efficiency can be obtained by measuring the lifetime. For obtaining the lifetime xcfx84n, conventional techniques defined the time for photoluminescence radiation to be attenuated from its maximum intensity to 1/e as the lifetime xcfx84n as explained in the above-introduced three patent applications.
However, since the lifetime xcfx84n depends upon the emitting lifetime xcfx84r and the emitting lifetime xcfx84r is inversely proportional to the excitation carrier density n, the lifetime xcfx84n also depends on the excitation carrier density n. Therefore, in order to compare different light emitting elements in life time and to discuss their emission efficiency, the premise that they are equal in excitation carrier density n is required.
On the other hand, recent improvements of epitaxial technologies using MOCVD enable accurate control of thickness and composition of thin films, and therefore contributed to realization of LED with a large excitation carrier density, such as visible high-luminescence LED having InGaAlP DH structure (double-heterostructure). As a result, it has become difficult to precisely estimate the emission efficiency by using conventional lifetime measuring methods configured to measure the lifetime without taking the excitation carrier density into consideration because the excitation carrier density was small. This is because excitation carrier density is large in high-luminescence LED, and its effect to the lifetime cannot be disregarded. Therefore, there has arisen the demand for a technique capable of measuring emission efficiency independently from excitation carrier density.
It is therefore an object of the invention to provide a evaluating method capable of evaluating an epitaxial wafer for a light emitting device, which is capable of quickly measuring the essential lifetime without breaking the wafer, independently from its excitation carrier density. A further object of the invention can be to provide an evaluating apparatus for such evaluation. A still further object of the invention is to provide an epitaxial wafer for a light emitting device, improved in emission efficiency. A still another object of the invention is to provide a recording medium readable by a computer to enable such evaluation.
According to the invention, there is provided an evaluating method for evaluating an epitaxial wafer for a light emitting device, comprising:
irradiating excited light onto the epitaxial wafer;
detecting photoluminescence radiation generated by excitation of carriers in an active layer of the epitaxial wafer; and
obtaining the non-radiative lifetime from the rate of changes in intensity of the photoluminescence radiation at the time when changes in intensity of the photoluminescence radiation with time become below a given value.
According to the invention, there is further provided a recording medium readable by a computer, and recording a program for the computer to execute:
a process for having excited light be irradiated onto an epitaxial wafer for a light emitting device;
a process for causing that intensity of photoluminescence radiation generated by excitation of carriers in an active layer of the epitaxial wafer be measured; and
a process for obtaining a non-radiative lifetime from the rate of changes in intensity of the photoluminescence radiation at the time when changes in intensity of the photoluminescence radiation with time become below a given value.
The xe2x80x9crecording mediumxe2x80x9d is any of hard disc (HD), DVD-RAM(digital versatile disc-random access memory), DVD-ROM(read only memory), magneto-optical recording medium, flexible disc (FD) and CD(compact disc)-ROM, or any of various memory devices such as RAM and ROM.
A program to be recorded on any of these mediums may be distributed through wire lines orwireless lines such as intranets and Internet in its original form or in a coded, modulated or compressed form, if necessary.
Further, the Inventors made researches on the relationship between the non-radiative lifetime measured by the above-summarized evaluating method and the diffusion amount of Zn into the active layer. As a result, it has been confirmed that, in case of an epitaxial wafer containing the diffusion amount of Zn not less than 1E13 atoms per cm2 in surface concentration, the non-radiative lifetime decreases as the Zn diffusion amount increases. Therefore, in order for an epitaxial wafer to ensure realization of LED with a high emission efficiency, the diffusion amount of Zn into its active layer has to be limited within 1E13 atoms/cm2, and its non-radiative lifetime must be 20 nanoseconds or longer. By using this wafer, LED with high luminescence and stable characteristics can be manufactured.
According to the invention, the true non-radiative lifetime in an active layer can be measured precisely and easily, independently from its excitation carrier density. As a result, epitaxial wafers having a high emission efficiency can be selected reliably, and the production yield is improved remarkably.
That is, according to the invention, in the stage of an epitaxial wafer, defective wafers and acceptable wafers can be discriminated. Therefore, outlet of defective wafers can be prevented, and the production yield is improved. For example, the ratio of defective wafers because of low luminescence is drastically reduced from 22% to 4% as shown in FIG. 8.
Additionally, according to the invention, an epitaxial wafer having a non-radiative lifetime not shorter than 20 nanoseconds can be obtained, and by limiting the diffusion amount of Zn into the active layer within 1E13 atoms per cm2, much better emission characteristics than conventional ones can be realized.
As explained above, the invention makes it easy to discriminate satisfactory wafers from defective wafers, or vice versa, and improved the production yield. Additionally, since the invention enables measurement of emission efficiency indispensable for fabrication of light emitting devices with a high accuracy, profitable industrial effects are expected.